new subaru plug wires or HIGH QUALITY replacements.
new NGK plugs.
fresh fuel, but if it smooths out after it runs for a while it probably isn't the fuel. fuel would still be iffy.
when you change the oil add a pint of MMO to the oil this will help loosen any crud in / on / around the HLAs, lash adjusters. these can get gummed up.t hen change the oil again in a couple of hundred miles.

let it idle for 30 - 40 minutes. and then drive it.

once you start driving it and get all the old fluids out it will improve. (assuming it was running ok when it was parked.) they usually do not go bad when sitting. at least nothing major.